Thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically, for example, from a color video camera or digital camera. An electronic picture can be subjected to color separation by color filters. The respective color-separated images can be converted into electrical signals. These signals can be operated on to produce cyan, magenta, and yellow electrical signals. These signals can be transmitted to a thermal printer. To obtain a print, a black, cyan, magenta, or yellow dye-donor layer, for example, can be placed face-to-face with a dye image-receiving layer of a receiver element to form a print assembly which can be inserted between a thermal printing head and a platen roller. A thermal print head can be used to apply heat from the back of the dye-donor sheet. The thermal print head can be heated up sequentially in response to the black, cyan, magenta, or yellow signals. The process can be repeated as needed to print all colors. A color hard copy corresponding to the original picture can be obtained. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 to Brownstein.
A problem exists with many of the dye-donor elements and dye image-receiving elements used in thermal dye transfer systems. At the high temperatures used for thermal dye transfer, many polymers used in these elements can soften and adhere to each other, resulting in sticking and tearing of the elements upon separation. Areas of the dye-donor layer (other than the transferred dye) can adhere to the dye image-receiving element, rendering the receiving element useless.
Many different means of solving donor-receiver sticking are known in the art. These include the addition of release agents to the dye-donor element or the receiver element. Use of silicone waxes and oils as lubricating elements are known in the art, as disclosed, for example, in JP 04-255394, and U.S. Pat. No. 4,643,917 to Koshizuka.
Use of dimethyl siloxanes is also known in the art. U.S. Pat. No. 5,356,859 to Lum et al. discloses the use of a dye image-receiving element including a polyoxyalkylene-modified dimethylsiloxane graft copolymer. However, the use of such material in high temperature extrusion coating of dye receiving layers is difficult because these materials degrade at high temperatures, resulting in unwanted reactions with other components of the dye-receiving layer. U.S. Pat. No. 4,839,338 to Marbrow discloses use of a receiver element coated with a release medium including two polysiloxanes which exhibit the same degradation and unwanted reactions as the materials in Lum. JP 61-262189 discloses the use of polyoxyalkylene silicone copolymers as a release material for use in heat sensitive recording materials, particularly where the polyoxyalkylene is grafted into the polysiloxane backbone for use in very high power printers. Release agents such as those listed above can affect the quality of the image printed on the dye image-receiving layer, and can be unsuitable for use in certain receiver elements, such as those prepared by extrusion coating of the dye image-receiving layer.
Various release agents have been found to exhibit decreased effectiveness when added to an extruded dye image-receiving layer. The release agents of the prior art can cause cross-linking and degradation of the extruded polymer mixture at the high temperatures used during extrusion, resulting in decreased image quality, such as the appearance of lines and streaks.
It is known to use phenol-based stabilizers and antioxidants, for example, alpha-tocopherol, to increase stability in extruded polyolefin compositions used for packaging material, for example, as described in U.S. Pat. Nos. 6,451,423, and 6,448,357. These compositions are not suitable for use in extruded thermal receivers.
There is a need in the art for a means to reduce or eliminate donor-receiver sticking, while reducing amounts of additives in a receiver element, and maintaining good print image formation. There is also a need to provide a receiver element capable of formation by extrusion coating to reduce costs. There is an additional need for stabilization, for example, at extrusion temperatures, in oxidative conditions, or both. Additives suitable for use in extruded thermal receivers are desirably capable of reducing donor-receiver sticking, improving coating quality, and are thermally stable.